* ld.texinfo (Builtin Functions) <DEFINED>: Say that only symbols

	* ld.texinfo (Builtin Functions) <DEFINED>: Say that only symbols
	defined before the statement using DEFINED yield 1.
	* ldexp.c (fold_name) <case DEFINED>: In lang_first_phase_enum,
	call lang_track_definedness on symbol.  In subsequent phases, use
	lang_symbol_definition_iteration and lang_statement_iteration to
	check whether the symbol was defined before the current statement.
	(exp_fold_tree) <case etree_assign et al>: Call
	lang_update_definedness before updating symbol type when setting
	symbol.
	* ldlang.c (lang_definedness_table): New variable.
	(lang_definedness_newfunc, lang_track_definedness)
	(lang_symbol_definition_iteration, lang_update_definedness): New
	functions.
	(lang_init): Initialize lang_definedness_table and
	lang_statement_iteration.
	(lang_finish): Destroy bfd_hash_table_free.
	(lang_size_sections): Increment lang_statement_iteration.
	(lang_do_assignments_1): New function with former
	lang_do_assignments contents.  Change recursive calls to call this
	function.
	(lang_do_assignments): Evacuate contents.  Increment
	lang_statement_iteration, then just call lang_do_assignments_1.
	* ldlang.h (struct lang_definedness_hash_entry)
	(lang_statement_iteration, lang_track_definedness)
	(lang_symbol_definition_iteration, lang_update_definedness):
	Declare.